ORISKANY SANDSTONE. ««• 



Cyrtia rostrata. 



Plate XCVI. Fia. 1 - 6 ; and Plate XCVIII. Pig. 8 a, J. 

 Cyrtia rostrata : Hall, Regents' Report for 1856, p. 64; Palaeozoic Fossils, 1857, p. 24. 



Shell triangularly pyramidal, more or less elevated : hinge-line straight, 

 equalling or greater than the greatest width of the shell below, and 

 sometimes extending into acute angles. Ventral valve much elevated 

 at the beak, a distinct sinus extending from beak to front : beak simple, 

 angular, not incurved. Dorsal valve convex, semicircular ; mesial fold 

 moderately elevated, rounded or slightly flattened, and marked with a 

 faint longitudinal depressed line : beak scarcely elevated above the 

 cardinal margin : area broad triangular, plane, or rarely subarcuate ; 

 foramen narrow, extending to the apex of the beak of the ventral valve, 

 partly closed above by a central plate. 



Spbface marked by five to eight or nine elevated subangular costse on 

 each side of the middle, concentrically marked by imbricating lamellae 

 and finer striae, which are crossed by fine radiating striae ; the entire 

 surface, in perfect specimens, being papillose. 



The interior of the dorsal valve shows two strong crural processes with 

 the dental fossets, and a cardinal process which is more or less developed. 

 In the ventral valve, the dental lamellae, uniting at their bases, are con- 

 tinued in a median septum, which is extended nearly to the anterior 

 margin of the shell : the same septum, extending into the triangular 

 cavity of the foramen, approaches to the plane of the area, which, in well- 

 preserved specimens, is distinctly striated vertically. A single specimen 

 preserves the internal spires. 



This species bears some resemblance to Cyrtia (Spirifer) heteroclita ; but it has 

 a greater number of plications, and is a larger and coarser shell. 



The youngest specimen observed of this species has six plications on each side of 

 the mesial fold, while the older ones have eight or nine : the plications vary from 

 angular to round. 



